U.S. patent number 6,912,413 [Application Number 10/661,012] was granted by the patent office on 2005-06-28 for pulse oximeter.
This patent grant is currently assigned to GE Healthcare Finland OY. Invention is credited to Aki Backman, Borje Rantala.
United States Patent |
6,912,413 |
Rantala , et al. |
June 28, 2005 |
**Please see images for:
( Certificate of Correction ) ** |
Pulse oximeter
Abstract
The invention relates to pulsed oximeters used to measure blood
oxygenation. The current trend towards mobile oximeters has brought
the problem of how to minimize power consumption without
compromising on the performance of the device. To tackle this
problem, the present invention provides a method for controlling
optical power in a pulse oximeter. The signal-to-noise ratio of the
received baseband signal is monitored, and the duty cycle of the
driving pulses is controlled in dependence on the monitored
signal-to-noise ratio, preferably so that the optical power is
minimized within the confines of a predetermined lower threshold
set for the signal-to-noise ratio. In this way the optical power is
made dependent on the perfusion level of the subject, whereby the
power can be controlled to a level which does not exceed that
needed for the subject.
Inventors: |
Rantala; Borje (Helsinki,
FI), Backman; Aki (Helsinki, FI) |
Assignee: |
GE Healthcare Finland OY
(FI)
|
Family
ID: |
31997955 |
Appl.
No.: |
10/661,012 |
Filed: |
September 12, 2003 |
Current U.S.
Class: |
600/322;
600/330 |
Current CPC
Class: |
A61B
5/14551 (20130101); A61B 2560/0209 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 005/00 () |
Field of
Search: |
;600/322-324,330 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Winakur; Eric F.
Assistant Examiner: Kremer; Matthew
Attorney, Agent or Firm: Marsh Fischmann & Breyfogle
LLP
Parent Case Text
RELATED APPLICATION
This application claims priority under 35 U.S.C. .sctn. 119 to
prior U.S. Provisional Patent Application No. 60/410,526, filed
Sep. 13, 2002, entitled "PULSE OXIMETER", the entire contents of
which are incorporated herein as if set forth herein in full.
Claims
What is claimed is:
1. A method for controlling optical power in a monitoring device
intended for determining the amount of at least one light absorbing
substance in a subject, the monitoring device comprising emitters
for emitting radiation at a minimum of two wavelengths driving
means for activating said emitters, and a detector for receiving
said radiation at said wavelengths and for producing an electrical
signal in response to the radiation, the method comprising the
steps of supplying driving pulses from said driving means to the
emitters, the pulses having predetermined characteristics
determining the optical power of the device, demodulating the
electrical signal originating from said detector whereby a baseband
signal is obtained; transforming the baseband signal into a
frequency spectrum to identify an amplitude and a noise level of
the baseband signal, whereby a signal-to-noise ratio of the
amplitude to the noise for the baseband signal is obtained;
monitoring the signal-to-noise ratio of the baseband signal, and
controlling the duty cycle of the driving pulses in dependence on
the monitored signal-to-noise ratio.
2. A method according to claim 1, wherein said controlling step
includes controlling the duty cycle of the driving pulses so that
the signal to noise ratio is maintained within the confines of a
predetermined range for the signal-to-noise ratio.
3. A method according to claim 2, wherein said controlling step
further includes comparing the monitored signal-to-noise ratio to
said predetermined range, said predetermined range being defined by
a predetermined lower threshold and a predetermined higher
threshold.
4. A method according to claim 3, further including the step of
connecting the electrical signal originating from said detector
through a preamplifier and a low-pass-filter prior to said
demodulating step.
5. A method according to claim 4, wherein said controlling step
includes performing at least one operation in response to said
signal-to-noise ratio reaching said lower threshold, the said at
least one operation being selected from a group of operations
including (1) the increase of the width of said pulses and (2) the
increase of pulse repetition rate, and decreasing the bandwidth of
said low-pass filter when the width of said pulses is
increased.
6. A method according to claim 5, wherein the controlling step
further includes the step of increasing the amplitude of said
driving pulses.
7. A method according to claim 4, wherein said controlling step
includes selecting at least one operation in response to said
signal-to-noise ratio reaching said higher threshold, the said at
least one operation being selected from a group of operations
including (1) the decrease of the width of said pulses and (2) the
decrease of pulse repetition rate, and increasing the bandwidth of
said low-pass filter when the width of said pulses is
decreased.
8. A method according to claim 7, wherein the controlling step
further includes the step of decreasing the amplitude of said
driving pulses.
9. A method according to claim 1, wherein said demodulating step
includes sampling of the electrical signal by a synchronous
detector, taking one sample per each pulse of the electrical
signal.
10. A method according to claim 1, wherein the amount of at least
one light absorbing substance is determined in the blood of a
subject.
11. A method according to claim 1, wherein the monitoring device is
a pulse oximeter.
12. An apparatus for non-invasively determining the amount of at
least one light absorbing substance in a subject, the apparatus
comprising emitters for emitting radiation at a minimum of two
different wavelengths, driving means for activating said emitters,
adapted to supply driving pulses to the emitters, the pulses having
predetermined characteristics determining current optical power of
the device, a detector for receiving said radiation at said
wavelengths and producing an electrical signal in response to the
radiation, a demodulator unit for demodulating the electrical
signal originating from the detector, whereby a baseband signal is
obtained from the demodulator unit, monitoring means for:
transfoming the baseband signal into a frequency spectrum;
generating a signal-to-noise ratio of the transformed baseband
signal; and monitoring the signal-to-noise ratio of the baseband
signal, and power control means, responsive to the monitoring
means, for controlling the duty cycle of the driving pulses.
13. An apparatus according to claim 12, wherein the power control
means are adapted to control the duty cycle so that the
signal-to-noise ratio is maintained within a predetermined range
between a first threshold and a second threshold.
14. An apparatus according to claim 13, further comprising a
low-pass filter for filtering said electrical signal prior to said
demodulating, the control means comprising at least one set of
first and second means, wherein the first means are adapted to
change the width of said pulses and of the passband of the low-pass
filter, and the second means are adapted to increase pulse
repetition rate.
15. An apparatus according to claim 14, wherein the control means
further comprise means for changing the amplitude of said
pulses.
16. An apparatus according to claim 13, wherein said apparatus is a
pulse oximeter.
17. A method for controlling optical power in a monitoring device
intended for determining the amount of at least one light absorbing
substance in a subject, the monitoring device comprising emitters
for emitting radiation at a minimum of two wavelengths, driving
means for activating said emitters, and a detector for receiving
said radiation at said wavelengths and for producing an electrical
signal in response to the radiation, the method comprising the
steps of supplying driving pulses from said driving means to the
emitters, the pulses having predetermined characteristics
determining the optical power of the device, demodulating the
electrical signal originating from said detector to generate
demodulated signals for said wavelengths; obtaining a DC signal
component for at least one of said demodulated signals; monitoring
a signal-to-noise ratio of the DC signal component, and controlling
the duty cycle of the driving pulses in dependence on the monitored
signal-to-noise ratio of the DC signal component.
18. A method according to claim 17, wherein said controlling step
includes controlling the duty cycle of the driving pulses so that
the signal to noise ratio is maintained within the confines of a
predetermined range for the signal-to-noise ratio.
19. A method according to claim 18, wherein said controlling step
further includes comparing the monitored signal-to-noise ratio to
said predetermined range, said predetermined range being defined by
a predetermined lower threshold and a predetermined higher
threshold.
20. A method according to claim 19, further including the step of
connecting the electrical signal originating from said detector
through a preamplifier and a low-pass-filter prior to said
demodulating step.
21. A method according to claim 20, wherein said controlling step
includes performing at least one operation in response to said
signal-to-noise ratio reaching said lower threshold, the said at
least one operation being selected from a group of operations
including (1) the increase of the width of said pulses and (2) the
increase of pulse repetition rate, and decreasing the bandwidth of
said low-pass filter when the width of said pulses is
increased.
22. A method according to claim 21, wherein the controlling step
further includes the step of increasing the amplitude of said
driving pulses.
23. A method according to claim 20, wherein said controlling step
includes selecting at least one operation in response to said
signal-to-noise ratio reaching said higher threshold, the said at
least one operation being selected from a group of operations
including (1) the decrease of the width of said pulses and (2) the
decrease of pulse repetition rate, and increasing the bandwidth of
said low-pass filter when the width of said pulses is
decreased.
24. A method according to claim 23, wherein the controlling step
further includes the step of decreasing the amplitude of said
driving pulses.
25. A method according to claim 17, wherein said demodulating step
includes sampling of the electrical signal by a synchronous
detector, taking one sample per each pulse of the electrical
signal.
26. A method according to claim 17, wherein the amount of at least
one light absorbing substance is determined in the blood of a
subject.
27. A method according to claim 17, wherein the monitoring device
is a pulse oximeter.
28. An apparatus for non-invasively determining the amount of at
least one light absorbing substance in a subject, the apparatus
comprising emitters for emitting radiation at a minimum of two
different wavelengths, driving means for activating said emitters,
adapted to supply driving pulses to the emitters, the pulses having
predetermined characteristics determining current optical power of
the device, a detector for receiving said radiation at said
wavelengths and producing an electrical signal in response to the
radiation, a demodulator unit for demodulating the electrical
signal originating from the detector to generate demodulated
signals for said wavelengths, whereby a DC signal component of at
least one of said demodulated signals is obtained from the
demodulator unit, monitoring means for monitoring a signal-to-noise
ratio of the DC signal component, and power control means,
responsive to the monitoring means, for controlling the duty cycle
of the driving pulses.
29. An apparatus according to claim 28, wherein the power control
means are adapted to control the duty cycle so that the
signal-to-noise ratio is maintained within a predetermined range
between a first threshold and a second threshold.
30. An apparatus according to claim 29, further comprising a
low-pass filter for filtering said electrical signal prior to said
demodulating, the control means comprising at least one set of
first and second means, wherein the first means are adapted to
change the width of said pulses and of the passband of the low-pass
filter, and the second means are adapted to increase pulse
repetition rate.
31. An apparatus according to claim 30, wherein the control means
further comprise means for changing the amplitude of said
pulses.
32. An apparatus according to claim 29, wherein said apparatus is a
pulse oximeter.
Description
FIELD OF THE INVENTION
The invention relates generally to devices used for non-invasively
determining the amount of at least one light absorbing substance in
a subject. These devices are typically pulse oximeters used to
measure the blood oxygenation of a patient. More specifically, the
invention relates to the optimization of power consumption in such
a device.
BACKGROUND OF THE INVENTION
Pulse oximetry is at present the standard of care for the
continuous monitoring of arterial oxygen saturation (SpO.sub.2).
Pulse oximeters provide instantaneous in-vivo measurements of
arterial oxygenation, and thereby provide early warning of arterial
hypoxemia, for example.
A pulse oximeter comprises a computerized measuring unit and a
probe attached to the patient, typically to his or her finger or
ear lobe. The probe includes a light source for sending an optical
signal through the tissue and a photo detector for receiving the
signal after transmission through the tissue. On the basis of the
transmitted and received signals, light absorption by the tissue
can be determined. During each cardiac cycle, light absorption by
the tissue varies cyclically. During the diastolic phase,
absorption is caused by venous blood, tissue, bone, and pigments,
whereas during the systolic phase, there is an increase in
absorption, which is caused by the influx of arterial blood into
the tissue. Pulse oximeters focus the measurement on this arterial
blood portion by determining the difference between the peak
absorption during the systolic phase and the constant absorption
during the diastolic phase. Pulse oximetry is thus based on the
assumption that the pulsatile component of the absorption is due to
arterial blood only.
Light transmission through an ideal absorbing sample is determined
by the known Lambert-Beer equation as follows:
where I.sub.in is the light intensity entering the sample,
I.sub.out is the light intensity received from the sample, D is the
path length through the sample, .epsilon. is the extinction
coefficient of the analyte in the sample at a specific wavelength,
and C is the concentration of the analyte. When I.sub.in, D, and
.epsilon. are known and I.sub.out is measured, the concentration C
can be calculated.
In pulse oximetry, in order to distinguish between the two species
of hemoglobin, oxyhemoglobin (HbO.sub.2), and deoxyhemoglobin
(RHb), absorption must be measured at two different wavelengths,
i.e. the probe includes two different light emitting diodes (LEDs).
The wavelength values widely used are 660 nm (red) and 940 nm
(infrared), since the said two species of hemoglobin have
substantially different absorption values at these wavelengths.
Each LED is illuminated in turn at a frequency which is typically
several hundred Hz.
The accuracy of a pulse oximeter is affected by several factors.
This is discussed briefly in the following.
Firstly, the dyshemoglobins which do not participate in oxygen
transport, i.e. methemoglobin (MetHb) and carboxyhemogiobin (CoHb),
absorb light at the wavelengths used in the measurement. Pulse
oximeters are set up to measure oxygen saturation on the assumption
that the patient's blood composition is the same as that of a
healthy, non-smoking individual. Therefore, if these species of
hemoglobin are present in higher concentrations than normal, a
pulse oximeter may display erroneous data.
Secondly, intravenous dyes used for diagnostic purposes may cause
considerable deviation in pulse oximeter readings. However, the
effect of these dyes is short-lived since the liver purifies blood
efficiently.
Thirdly, coatings such as nail polish may in practice impair the
accuracy of a pulse oximeter, even though the absorption caused by
them is constant, not pulsatile, and thus in theory it should not
have any effect on the accuracy.
Fourthly, the optical signal may be degraded by both noise and
motion artifacts. One source of noise is the ambient light received
by the photodetector. Many solutions have been devised with the aim
of minimizing or eliminating the effect of the movement of the
patient on the signal, and the ability of a pulse oximeter to
function correctly in the presence of patient motion depends on the
design of the pulse oximeter. One way of canceling out the motion
artifact is to use an extra wavelength for this purpose.
One of the current trends in pulse oximetry is the aim towards
lower power consumption, which is essential for battery-operated
oximeters, for example. These oximeters are typically mobile and
must therefore be used in various locations where both the
characteristics of the patient and the surrounding measurement
environment may vary. A problem related to these various
measurement conditions is the optimization of power consumption
without compromising the performance of the device, i.e. how to
guarantee reliable measurement results even in difficult
measurement conditions and still keep the battery life as long as
possible.
The current straightforward solution for obtaining reliable
measurement results under tough measurement conditions is to
increase the driving power of the LEDs. This approach is based on
the transmittance of the tissue: if the level of the signal
transmitted through the tissue is not enough to guarantee reliable
results, the level of the transmitted signal (i.e. the amplitude of
the pulse train) is increased until the level of the signal
received is sufficient. This is naturally contrary to the need to
save power.
It is an objective of the invention to bring about a solution by
means of which it is possible to dynamically optimize the power
consumption in a pulse oximeter, especially in a portable
battery-operated pulse oximeter, and to maintain good performance
even in tough measurement conditions, where the transmittance
and/or the perfusion level, as indicated by the normalized
pulsatile component, are low.
SUMMARY OF THE INVENTION
These and other objectives of the invention are accomplished in
accordance with the principles of the present invention by
providing a power-saving scheme which allows the pulse oximeter to
use no more power than that which is needed to drive the emitters
while maintaining good performance of the oximeter. In this scheme,
the signal-to-noise requirements are compromised in favor of power
consumption, as long as this does not compromise measurement
reliability.
According to the invention, the patient-specific effect of the
tissue on the measurement result is taken into account, whereby the
optical power, i.e. the power supplied to activate the emitters,
can be controlled to a level which is no more than what is needed
for each measurement. The idea behind the invention is that the
measurement results of the pulse oximeter depend on the perfusion
level and on the transmittance of the tissue under illumination.
Therefore, the optical power in the pulse oximeter of the invention
can be controlled at each measurement occasion to a level which is
the minimum sufficient for the patient in question, by monitoring
the demodulated baseband signal indicative of the perfusion level
of the patient and controlling the duty cycle of the driving pulses
so that the optical power is minimized given a predetermined lower
threshold set for the signal-to-noise ratio.
The invention provides other ways to reach the level which is
enough at each time, said ways being applicable alone or in
combination. These ways can also be combined with the normal
increase of the amplitude of the driving pulses.
Under favorable conditions the requirements for the signal-to-noise
ratio of the pulse oximeter are eased in favor of power
consumption, and the optical power is dropped to the minimum level
sufficient for measurement. Power is then increased only when the
minimum signal-to-noise ratio ensuring a reliable measurement is
not otherwise reached.
Thus, one aspect of the invention is providing a method for
controlling optical power in a monitoring device intended for
determining the amount of at least one light absorbing substance in
a subject, the monitoring device comprising emitters for emitting
radiation at a minimum of two wavelengths, driving means for
activating said emitters, and a detector for receiving said
radiation at said wavelengths and for producing an electrical
signal in response to the radiation, the method comprising the
steps of supplying driving pulses from said driving means to the
emitters, the pulses having predetermined characteristics
determining the optical power of the device, demodulating the
electrical signal originating from said detector, whereby a
baseband signal is obtained, monitoring a signal-to-noise ratio of
the baseband signal, and controlling the duty cycle of the driving
pulses in dependence on the monitored signal-to-noise ratio.
The duty cycle is preferably controlled so that the optical power
is minimized within the confines of a predetermined lower threshold
set for the signal-to-noise ratio.
Another aspect of the invention is that of providing an apparatus
for non-invasively determining the amount of at least one light
absorbing substance in a subject, the apparatus comprising emitters
for emitting radiation at a minimum of two different wavelengths,
driving means for activating said emitters, adapted to supply
driving pulses to the emitters, the pulses having predetermined
characteristics determining current optical power of the device, a
detector for receiving said radiation at said wavelengths and
producing an electrical signal in response to the radiation, a
demodulator unit for demodulating the electrical signal originating
from said detector, whereby a baseband signal is obtained from the
demodulator unit, monitoring means for monitoring a signal-to-noise
ratio of the baseband signal, and power control means, responsive
to the monitoring means, for controlling the duty cycle of the
driving pulses.
The power control means are preferably adapted to perform the
controlling of the duty cycle so that the optical power is
minimized within the confines of a predetermined lower threshold
set for the signal-to-noise ratio.
The power control scheme of the present invention provides good
performance even when the tissue is thick (requiring high drive
current) and the perfusion (pulsatility) is low. In this case the
front stage of the pulse oximeter tends to saturate, whereby a
conventional pulse oximeter can no longer operate in a reliable
way. In contrast, the pulse oximeter of the invention may still
obtain reliable readings by widening the pulses or increasing the
pulse repetition rate, thereby increasing the signal-to-noise
ratio.
Other features and advantages of the invention will become apparent
by reference to the following detailed description and accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention and its preferred embodiments are
described more closely with reference to the examples shown in FIG.
1 to 4d in the appended drawings, wherein:
FIG. 1 illustrates a typical embodiment of a pulse oximeter
according to the present invention,
FIG. 2 is a flow diagram illustrating one embodiment of the power
control scheme of the present invention,
FIG. 3a illustrates the timing sequence of the detector signal when
a high duty cycle pulse sequence is used,
FIGS. 3b and 3c illustrate the frequency spectrum of a detector
signal according to FIG. 3a,
FIG. 3d illustrates the spectrum of the baseband signal obtained
from the signal of FIG. 3b after demodulation,
FIG. 4a illustrates the timing sequence of the detector signal when
a low duty cycle pulse sequence is used,
FIGS. 4b and 4c illustrate the frequency spectrum of a detector
signal according to FIG. 4a, and
FIG. 4d illustrates the spectrum of the baseband signal obtained
from the signal of FIG. 4b after demodulation.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram of one embodiment of a pulse oximeter
according to the present invention. This embodiment is based on a
traditional pulse oximeter where synchronous detection is used. At
least two different LEDs, A and B, are driven by a LED drive 10.
Each LED operates at a respective wavelength, and the light emitted
by the LEDs passes into patient tissue, such as a finger 11. The
light propagated through or reflected from the tissue is received
by a probe 12 including a photodetector. The photodetector converts
the optical signal received into an electrical signal and supplies
it to an amplifier stage 13, which includes a controllable
preamplifier 13a and a variable low-pass filter 13b. After the
amplifier stage, an analog switch 14, controlled by the control
unit 18, ensures that the signal is zeroed between consecutive
pulses, thereby removing background light. The reception branch is
then divided into two branches: the IR branch for the infrared
signal and the R branch for the red signal. Each branch is preceded
by an analog switch (not shown in the figure), which is controlled
by the control unit 18 so that the pulses are connected to their
respective branch (the R pulses to the R branch and the IR pulses
to the IR branch). In each branch a sampling unit (15, 16) then
takes samples of the pulses received by the branch. The control
unit controls the R sampling unit so that it samples the R pulses
and the IR sampling unit so that it samples the IR pulses. The
sampling units typically include a sampling switch and a capacitor
charged to the pulse voltage prevailing at the sampling moment. The
sampled signals are then supplied to an A/D converter 17, which
converts them into digitized format for the control unit 18. The
synchronous detection performed in the sampling units 15 and 16 is
also termed "demodulation" in this context, since it is the
operation which extracts the original modulating signal from the
detector signal.
In order to introduce the power-controlling scheme of the invention
into a pulse oximeter of the type shown in FIG. 1, the pulse
oximeter structure is modified so that the control unit 18 monitors
the baseband signal-to-noise ratio (i.e. the signal-to-noise ratio
of the demodulated baseband signal) and selects the optical power
in dependence on the monitored ratio. The power consumption is
minimized dynamically, so that when the monitored signal-to-noise
ratio is low, the control unit starts to compromise on power
consumption in favor of performance, thereby ensuring reliable
measurement results. As discussed below, minimizing power
consumption involves changing at least one parameter of the duty
cycle of the pulse train driving the LEDs so that the optical power
changes in the desired direction. The parameters include the pulse
width and the pulse repetition rate. When the control unit
decreases the pulse width, it simultaneously widens the bandwidth
of the low-pass filter 13b to allow the pulse to reach essentially
its full height. When the control unit increases the pulse width,
it simultaneously narrows the bandwidth of the low-pass filter to
decrease the amount of input noise. In addition to the pulse width
and/or the repetition rate, the pulse amplitude can also be
controlled.
FIG. 2 illustrates one embodiment of the power control scheme. It
is assumed here that the power control scheme is implemented in the
pulse oximeter of FIG. 1. As discussed above, the control unit
first defines the signal-to-noise ratio of the demodulated baseband
signal (step 21) and compares the ratio to a first threshold, which
defines the lower limit of an acceptable signal-to-noise ratio
(step 22). If the current ratio is below the first ratio, the
control unit increases the optical power by changing the duty cycle
of the pulse train (step 23), and the process returns to step 21 to
define the signal-to-noise ratio associated with the new
characteristics.
If it is detected at step 22 that the signal-to-noise ratio is
above the first threshold, it is examined at step 24 whether the
signal-to-noise ratio is also above the second threshold, which is
slightly higher than the first threshold. If this is not the case,
but the ratio is between the first and second thresholds, the
current characteristics of the pulse train are maintained, i.e. the
optical power is maintained at its current value (step 25). If it
is detected at step 24 that the signal-to-noise ratio is above a
second threshold, the duty cycle of the pulse train is changed at
step 26 so that the optical power is decreased. The process then
returns to step 21 to define the signal-to-noise ratio associated
with the new duty cycle of the pulse train.
The optical power can be increased in several ways at step 23. The
first method is to increase the pulse width, while simultaneously
decreasing the bandwidth of the low-pass filter 13b, which thereby
decreases the amount of input noise. The second method is to
increase the pulse repetition rate in order to decrease noise
aliasing, i.e. to decrease the number of harmonics being
down-converted by the synchronous demodulation. In addition to
these operations, the current or voltage of the pulses driving the
LEDs can be increased.
Accordingly, the optical power can be decreased in several ways at
step 26, whenever it is detected that the signal-to-noise
requirements can easily be met. The first method is to narrow the
pulses, simultaneously increasing the bandwidth of the low-pass
filter 13b, thereby allowing the pulses to reach approximately
their full height. The second method is to use a lower pulse
repetition rate, which allows more aliasing of interference/noise
in the demodulation phase due to a lower sampling rate and thus
degrades the signal-to-noise ratio on the baseband. The above
operations can be used alone or in combination to decrease the
optical power. In addition to these operations, the current or
voltage of the pulses driving the LEDs can be decreased. It is to
be understood that steps 23 and 26 include the control of the
bandwidth associated with the control of the pulse width.
FIG. 3a to 3d illustrate noise aliasing in a conventional high duty
cycle oximeter which uses LED pulses having a duty cycle greater
than 10%. The power control scheme of the present invention uses a
high duty cycle pulse train only when the desired signal-to-noise
ratio cannot otherwise be reached, i.e. the situation of FIG. 3a to
3d is entered at step 23 in FIG. 2. FIG. 4a to 4d correspond to
FIG. 3a to 3d, respectively, except that in FIG. 4a to 4d the pulse
oximeter is a narrow pulse oximeter where the LEDs are activated as
briefly as possible in order to save power. This power saving mode
is entered whenever conditions permit easing the signal-to-noise
requirements in favor of power consumption. As to the example of
FIG. 2, the power saving mode is entered in step 26, and the mode
is maintained in step 25.
It is assumed here that (1) in the high duty cycle mode the pulse
width equals 200 .mu.s and the pulse repetition rate f.sub.r is
equal to 1 kHz, i.e. the time period between two consecutive pulses
is 1 ms, and (2) in the power saving mode the pulse width is equal
to 20 .mu.s and the pulse repetition rate f.sub.r equals 100 Hz.
FIGS. 3a and 4a show the timing sequences of the detector signal in
the respective modes, whereas FIGS. 3b and 4b illustrate the
frequency spectrum of the detector signal in the respective modes.
FIGS. 3a and 4a also show the amplitude modulation appearing in the
pulse train at the heart rate of the patient. FIGS. 3c and 4c show
in more detail the part of the spectrum denoted by circles A in
FIGS. 3b and 4b, respectively.
As can be seen from FIGS. 3b, 3c, 4b, and 4c, the spectrum
comprises a main peak at the pulse repetition frequency and
harmonic peaks at the odd harmonic frequencies of the repetition
rate. Side peaks SP caused by the above-mentioned amplitude
modulation appear around the main and harmonic peaks. The frequency
deviation between a side peak and the associated main or harmonic
peak corresponds to the heart rate, which is in this context
assumed to be 1 Hz.
FIGS. 3d and 4d illustrate the frequency spectrum of the baseband
signal in the above-mentioned two modes, i.e. the frequency
spectrum of the signal after synchronous detection. The aliased
peaks contribute to the amplitude A1 of the signal at the heart
rate, whereas the surrounding noise level A2 is determined by the
noise aliased on the whole band (FIG. 3d). The amplitude of the
baseband signal (A1) indicates the perfusion level of the patient,
but the quantity to be controlled is the baseband signal-to-noise
ratio, which is directly dependent on the signal amplitude, i.e. on
the perfusion level.
As to the power saving mode of FIG. 4a to 4d, narrowing the pulses
and lowering their repetition rate has two consequences: the narrow
pulses require the preamplifier to have a wide bandwidth, and the
harmonic content of the detector signal is high (cf. FIG. 4b). When
demodulating the narrow pulses, all harmonic components of the
sampler are folded into the baseband. Therefore the noise level
(A2) on the baseband (FIG. 4d) is higher than in the high duty
cycle (FIG. 3d). In a sense the pulse harmonics belong to the
"payload signal", since they contribute to the amplitude A1 of the
signal at the heart rate, whereas the noise coming from the
detector, the preamplifier, or other sources do not.
The dashed lines in FIGS. 3b and 4b illustrate the passband of the
low-pass filter 13b contained in the amplifier stage, the passband
being controlled by the control unit 18 in the above-described
manner in association with the control of the pulse width. The
actual width of the passband depends on many factors. However, the
passband width is always kept at a value which allows the reception
of a sufficient amount of pulse energy. As can be seen in FIG. 3c
and 4c, the wider the pulses are the steeper the decline in
harmonic amplitude.
It was assumed above that the pulse oximeter is a conventional
pulse oximeter based on synchronous detection in the sampling
units. However, the power control scheme of the invention can also
be used with other types of pulse oximeters, for example, in a
known oximeter based on fast A/D conversion.
Although the invention was described above with reference to the
examples shown in the appended drawings, it is obvious that the
invention is not limited to these, but may be modified by those
skilled in the art without departing from the scope and spirit of
the invention. For example, the pulse oximeter can be provided with
more than two wavelengths and with auxiliary means for eliminating
external interference, such as motion artifact. The number of
distinct power levels depends on the implementation and can vary to
a great extent. For example, the number of possible pulse width
values depends on the resolution of the pulse width modulator used.
Furthermore, the method can also be used in devices other than
pulse oximeters, devices measuring other substances in a similar
manner, i.e. non-invasively by radiating the patient. An example of
such measurement is non-invasive optical monitoring of glucose or
bilirubin, or simply an optical pulse rate monitor.
* * * * *